1. Field of the Invention
The present invention relates to a method of fabricating a polycrystalline silicon film and more particularly to a method that uses a laser annealing process and has a larger crystal grain size of a polycrystalline silicon film than normal.
2. Description of Related Art
The major material used in semiconductors is silicon, and many forms of silicon are used, such as amorphous, polycrystalline and single crystalline silicon. Polycrystalline silicon thin film has recently attracted considerable attention due to its special physical features and low cost. Consequently, companies fabricating thin film transistor liquid crystal displays (TFT-LCD), TFT-Organic Light Emitting Displays (TFT-OLED) or silicon solar cells are interested in developments of this material.
Polycrystalline silicon is an aggregate of single crystal grains, and thus there are many grain boundaries between crystal grains. These grain boundaries are a scattering source for a carrier. Therefore, enlargement of crystal grain size is very important for high performance devices. The conventional methods of fabricating typical polycrystalline silicon film follow.
1. Solid phase crystallization (SPC).
2. Vapor phase deposition (VPD).
3. Laser annealing.
In the SPC and VPD methods, the crystal grain size is as small as 100 nm. Therefore, the electrical performance of the resultant poly-Si film is poor. In the laser annealing method, an excimer laser is a most typical light source due to its high light energy density. In the excimer laser annealing (ELA) method, an intense light beam of an ultra-short duration can heat the precursor silicon film up to its melting point to be crystallized during its solidification process while keeping the non-heat-resistant glass or plastic substrate at sufficiently low temperatures. Therefore, good electrical properties are expected in ELA fabricated poly-Si film because of its high process temperature for Si film. However, the drawback of the ELA method is that the crystal grain size is smaller than 0.3 xcexcm in a 100 nm-thick silicon film.
In the ELA method, the silicon film is melted by laser light, and then the silicon crystallizes during its solidification process. The longer the melt duration is, the larger crystal grain size is. If thermal flow from the molten silicon to the substrate can be suppressed, the crystal grain size can be enlarged. Several techniques are available to keep the heat in the molten silicon, such as supplying a low specific heat film between the silicon film and the substrate (W. Yeh et. al., Jpn. J. Appl. Phys., 38 (1999) L110) or heating the substrate to decrease the temperature gradient between the subtrate and the silicon film to slow down the heat flow (H. Kuriyama et. al. Jpn. J. Appl. Phys., 30 (1991) 3700; J. S. Im et al., Appl. Phys. Lett. 64 (1994) 2303). Materials with sufficiently lo specific heat are porous materials, but porous materials are not only difficult to fabricate but also have low alkali-resistance and/or acid-resistance. Therefore, porous film is usually destroyed in the back end processes for fabrication of devices.
To increase the grain size of silicon film in the ELA method, heating the substrate seems better for improving the large crystal grain size. The crystal grain size in ELA with 800xc2x0 C. substrate heating is two times larger than the crystal grain size at room temperature. However, the temperature in the substrate heating technique is restricted by the strain temperature of the substrate material. For example, 500xc2x0 C. is the upper limit for a glass substrate, and 150xc2x0 C. is the upper limit for a plastic substrate. The effect of substrate heating on crystal grain size at these temperatures is slight.
To overcome the shortcomings, the present invention provides a method to fabricate silicon film with large crystal grain size to mitigate and obviate the aforementioned problems.
The main objective of the present invention is to provide a method for fabricating a polycrystalline silicon film with large crystal grains.
Other objectives, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.